1. Field of the Invention
The invention relates to a polymer composed of
from 50 to 99.98% by weight of a C.sub.1 -C.sub.12 alkyl (meth)acrylate a), PA1 from 0.02 to 1.8% by weight of a vinyl-aromatic compound b), PA1 from 0 to 10% by weight of an ethylenically unsaturated acid or an ethylenically unsaturated acid anhydride c), and PA1 from 0 to 40% by weight of other monomers d), the percentages by weight being based on the polymer. PA1 by weight of a C.sub.1 -C.sub.12 alkyl (meth)acrylate a) PA1 by weight of a vinyl-aromatic compound b) PA1 weight of an ethylenically unsaturated acid c) PA1 weight of other monomers d) which are different from a) to c)
Furthermore, the invention relates to the use of the polymers or their aqueous dispersions.
2. Description of the Background
Pressure-sensitive adhesives (PSAs) form a permanent tacky film which at room temperature, even under slight pressure, sticks to a very wide variety of surfaces. Pressure-sensitive adhesives are used to produce self-adhesive products such as self-adhesive labels, tapes and films. Products of this kind are very simple to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation times are necessary. Moreover, there is no "open time" within which the adhesive bond must be implemented. The quality of a self-adhesive article depends essentially on whether the internal strength (cohesion) and the sticking of the adhesive film on the surface that is to be bonded (adhesion) are set properly in relation to one another in accordance with the utility.
In the case of pressure-sensitive adhesives for labels, in particular, the level of cohesion must be sufficient for no stringing and no emergence of glue at the edges to occur in the course of stamping and cutting, since otherwise the cutting tools become soiled and the cut faces sticky. At the same time, the adhesion should be at a high level in order to obtain good sticking on the substrate that is to be bonded.
In general it is impossible to optimize adhesion and cohesion independently of one another. There is a desire for measures which either lift the level of both properties or, at least, maintain one property unchanged while improving the other.
Adhesives, including pressure-sensitive adhesives, based on polymers which can contain styrene are already known from DE-A-3 013 812, U.S. Pat. No. 5,416,134 and EP-A-625 557. The polymers prepared or used in the examples of these disclosures, however, contain at least 4% by weight styrene. Copolymers of this kind still do not have the desired level of adhesion and cohesion or the desired ratio of adhesion to cohesion.